netdev - Re: [PATCH V7 6/9] PCI: Enable 10-Bit Tag support for PCIe RP devices

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Date:   Thu, 5 Aug 2021 16:25:23 +0800
From:   Dongdong Liu <liudongdong3@...wei.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
CC:     <hch@...radead.org>, <kw@...ux.com>, <logang@...tatee.com>,
        <leon@...nel.org>, <linux-pci@...r.kernel.org>,
        <rajur@...lsio.com>, <hverkuil-cisco@...all.nl>,
        <linux-media@...r.kernel.org>, <netdev@...r.kernel.org>
Subject: Re: [PATCH V7 6/9] PCI: Enable 10-Bit Tag support for PCIe RP devices

On 2021/8/5 7:38, Bjorn Helgaas wrote:
> On Wed, Aug 04, 2021 at 09:47:05PM +0800, Dongdong Liu wrote:
>> PCIe spec 5.0r1.0 section 2.2.6.2 implementation note, In configurations
>> where a Requester with 10-Bit Tag Requester capability needs to target
>> multiple Completers, one needs to ensure that the Requester sends 10-Bit
>> Tag Requests only to Completers that have 10-Bit Tag Completer capability.
>> So we enable 10-Bit Tag Requester for root port only when the devices
>> under the root port support 10-Bit Tag Completer.
>
> Fix quoting.  I can't tell what is from the spec and what you wrote.
Will fix.
>
>> Signed-off-by: Dongdong Liu <liudongdong3@...wei.com>
>> ---
>>  drivers/pci/pcie/portdrv_pci.c | 69 ++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 69 insertions(+)
>>
>> diff --git a/drivers/pci/pcie/portdrv_pci.c b/drivers/pci/pcie/portdrv_pci.c
>> index c7ff1ee..2382cd2 100644
>> --- a/drivers/pci/pcie/portdrv_pci.c
>> +++ b/drivers/pci/pcie/portdrv_pci.c
>> @@ -90,6 +90,72 @@ static const struct dev_pm_ops pcie_portdrv_pm_ops = {
>>  #define PCIE_PORTDRV_PM_OPS	NULL
>>  #endif /* !PM */
>>
>> +static int pci_10bit_tag_comp_support(struct pci_dev *dev, void *data)
>> +{
>> +	bool *support = (bool *)data;
>> +
>> +	if (!pci_is_pcie(dev)) {
>> +		*support = false;
>> +		return 1;
>> +	}
>> +
>> +	/*
>> +	 * PCIe spec 5.0r1.0 section 2.2.6.2 implementation note.
>> +	 * For configurations where a Requester with 10-Bit Tag Requester
>> +	 * capability targets Completers where some do and some do not have
>> +	 * 10-Bit Tag Completer capability, how the Requester determines which
>> +	 * NPRs include 10-Bit Tags is outside the scope of this specification.
>> +	 * So we do not consider hotplug scenario.
>> +	 */
>> +	if (dev->is_hotplug_bridge) {
>> +		*support = false;
>> +		return 1;
>> +	}
>> +
>> +	if (!(dev->pcie_devcap2 & PCI_EXP_DEVCAP2_10BIT_TAG_COMP)) {
>> +		*support = false;
>> +		return 1;
>> +	}
>> +
>> +	return 0;
>> +}
>> +
>> +static void pci_configure_rp_10bit_tag(struct pci_dev *dev)
>> +{
>> +	bool support = true;
>> +
>> +	if (dev->subordinate == NULL)
>> +		return;
>> +
>> +	/* If no devices under the root port, no need to enable 10-Bit Tag. */
>> +	if (list_empty(&dev->subordinate->devices))
>> +		return;
>> +
>> +	pci_10bit_tag_comp_support(dev, &support);
>> +	if (!support)
>> +		return;
>> +
>> +	/*
>> +	 * PCIe spec 5.0r1.0 section 2.2.6.2 implementation note.
>> +	 * In configurations where a Requester with 10-Bit Tag Requester
>> +	 * capability needs to target multiple Completers, one needs to ensure
>> +	 * that the Requester sends 10-Bit Tag Requests only to Completers
>> +	 * that have 10-Bit Tag Completer capability. So we enable 10-Bit Tag
>> +	 * Requester for root port only when the devices under the root port
>> +	 * support 10-Bit Tag Completer.
>> +	 */
>> +	pci_walk_bus(dev->subordinate, pci_10bit_tag_comp_support, &support);
>> +	if (!support)
>> +		return;
>> +
>> +	if (!(dev->pcie_devcap2 & PCI_EXP_DEVCAP2_10BIT_TAG_REQ))
>> +		return;
>> +
>> +	pci_dbg(dev, "enabling 10-Bit Tag Requester\n");
>> +	pcie_capability_set_word(dev, PCI_EXP_DEVCTL2,
>> +				 PCI_EXP_DEVCTL2_10BIT_TAG_REQ_EN);
>> +}
>> +
>>  /*
>>   * pcie_portdrv_probe - Probe PCI-Express port devices
>>   * @dev: PCI-Express port device being probed
>> @@ -111,6 +177,9 @@ static int pcie_portdrv_probe(struct pci_dev *dev,
>>  	     (type != PCI_EXP_TYPE_RC_EC)))
>>  		return -ENODEV;
>>
>> +	if (type == PCI_EXP_TYPE_ROOT_PORT)
>> +		pci_configure_rp_10bit_tag(dev);
>
> I don't think this has anything to do with the portdrv, so all this
> should go somewhere else.
Yes, any suggestion where to put the code?
>
> Out of curiosity, IIUC this enables 10-bit tags for MMIO transactions
> from the root port toward the device, i.e., traffic that originates
> from a CPU.  Is that a significant benefit?  I would expect high-speed
> devices would primarily operate via DMA with relatively little MMIO
> traffic.
The benefits of 10-Bit Tag for EP are obvious.
There are few RP scenarios. Unless there are two:
1. RC has its own DMA.
2. The P2P tag is replaced at the RP when the P2PDMA go through RP.

Thanks,
Dongdong
>
>>  	if (type == PCI_EXP_TYPE_RC_EC)
>>  		pcie_link_rcec(dev);
>>
>> --
>> 2.7.4
>>
> .
>